Method for evaporating subliming materials



June 18, 1963 M. E. RICHARDSON METHOD FOR EVAPORATING SUBLIMING MATERIALS Filed Jan. 12, 1959 TO VACUUM PUMP INVENTOR. MORRIS E. RICHARDSON ATTORNEY Patented June 18, 1963 [ice 3,094,295 lvmrnou son EVAPGRATiNG some MATELS Morris E. Richardson, Rochester, N.Y., assignor to General Dynamics Corporation, Rochester, N.Y., a corporation of Delaware Filed Jan. 12, 1959, Ser. No. 786,395 6 Qlaims. (Cl. 225-294) This invention rel-ates to evaporation techniques.

Materials which are known to sublime, i.e., change directly from a solid state to a gaseous state and vice versa without apparently liquifying, have been used as coating materials. In general, the coating process involves the step of evaporating the subliming material and thereafter condensing the vapor of sublimation upon the surface to be coated. Some types of such materials may be more efiectively caused to sublime in a partial vacuum in order to minimize chemical reactions and decomposition and in order to expedite the processes involved. These subliming techniques may be referred to generally as vacuum evaporation techniques. An example of one material that may be coated on a desired surface by vacuum evaporation techniques is cadmium sulfide. Many other substances may be susceptible to effective use of evaporation coating techniques with or without a partial vacuum or, alternatively, in a particular gaseous atmosphere at a particular pressure.

It is a principal object of the invention to provide an improved method for evaporating particulate subliming material.

Another important object of the invention is to provide an improved method for evaporating a particulate subliming material in a partial vacuum.

Yet another object of the invention is to provide an improved method for evaporating particulate subliming material such that the sputtering of the evaporating material is minimized so that a uniform coating deposit of condensed vapors of sublimation may be obtained on the surface of an object placed near the subliming material.

The method of the invention features the step of placing a pervious layer of inert particulate material over the particulate material to be sublimed in a manner to quiet the physical movements of the sub-liming material which are customarily produced when the material su'blimes. In such manner, the jumping and sputtering of the subliming material is suppressed so that an object to be coated with condensed sublimation vapors placed near the subliming material will not be showered with particulate subliming material.

Further objects, features, and the attending advantages of the invention will be apparent with reference to the following specification and drawing which is a diagrammatic illustration of one simple type of apparatus which may be used in practicing a preferred method of the invention.

As briefly mentioned above, it has been found that when a particulate subliming material is caused to evaporate, quite vigorous physical movements are imparted to the particulate material causing the material to jump and spatter out of its container as it evaporates. It is known to coat surfaces of articles with condensed vapors of sublimation and in order to do so, the surface to be coated is placed in the vicinity of an evaporation vessel containing the particulate sub-liming material. Ordinarily, the evaporation vessel, which may be a crucible or the like, is heated to a requisite temperature to cause the particulate material to sublime and, obviously, different types of subliming materials have different ran es of temperature for effective sublimation and evaporation. Also, various particulate subliming materials may sub-lime most effectively in a particular gaseous atmosphere at a particular pressure or, alternatively, may sublime most effectively in a partial atmospheric vacuum.

According to the method of the invention, the physical movements of the particulate subliming material as it evaporates are suppressed by placing a pervious layer of inert particulate material over the layer of particulate sub'li-ming material. The pervious layer should be of a requisite thickness to elfectively suppress the movements of the sublimin-g mate-rial and also the perv-ious layer material should have a chemical composition that Will not decompose, liquify, vaporize, or otherwise deteriorate or chemically associate with the subliming material under the temperature and pressure conditions of the sublimation processes. It is also preferable that the particulate inert material have a particle size at least greater than the particle size of the particulate subliming material although the invention is not limited to the use of a specified particle size for the particulate inert material which may even be smaller in some cases. With the pervious layer of particulate material placed over the particulate subiiming material in the evaporation container, the subliming material, as it evaporates, is believed to be caused to condense and I'E-EVEPOIZltfi a plurality of times on the particulate inert material as it percolates through the pervious layer of particulate inert material in the process of evaporation into the surrounding gaseous atmosphere. Whether or not the surrounding gaseous atmosphere is maintained at atmospheric pressure or at greater or lower than atmospheric pressure is not important to the teachings of the present invention since various types of subliming materials will display the objectionable sputtering and spattering motions during evaporation to greater or lesser extents under most of the various conditions of evaporation.

The production of photoelectric film surfaces on conductive glass may require the deposit of a uniform thin film of cadmium sulfide on the requisite glass surface. One of the known techniques for depositing a film of cadmium sulfide on a requisite surface is the vacuum evaporation coating technique which takes advantages of the fact that particulate cadmium sulfide will sublime directly from the solid to the gaseous state and. will condense on the surface of articles placed in contact with the vapor of sublimation. Preferably, the particulate c-admium sulfide material is evaporated in a partial vacuum.

Referring to the drawing, one form of apparatus for vacuum evaporating particulate material with the novel method of the present invention is disclosed. A vacuum table 19 is provided with a conduit 11 connecting through table '10 between its upper surface and a vacuum pump. A glass vacuum jar, commonly referred to as a bell jar 12, may be placed on the vacuum table, as shown, so that the operation of the vacuum pump will produce a partial vacuum within the enclosure provided by the vacuum table 10 and the bell jar 12. Positioned within such enclosure may be mounted a crucible 15 to contain a layer 16 of particulate subliming material covered by a pervious layer of inert particulate material 17. A resistance heating coil 18 surrounds the crucible 15 in order to heat the crucible to a desired temperature upon application of electric power at the terminals 19 and 20 that are externally connected. It should be understood that the invention is not limited to a particular arrangement for heating the crucible or container 15. A sheet 21 of material to be coated with the vapor of sublimation may be supported in any suitable manner within the enclosure and, for example, may be supported on the arms 22 and '23 in a position over the mouth of crucible 15 so that the vapors issuing therefrom may impinge on the undersurface of the sheet 2.1 and condense thereon. It should be understood, however, that the method of this invention is not concerned with the manner of placement for the article to be coated since the article may be placed in various positions within the container depending upon the desired coating effects to be obtained. Furthermore, the method of the invention is directed primarily to the technique of evaporating a particulate subliming material and is not concerned with the use to which the vapors of sublimation may be put as they are formed.

When the particulate subliming material 16 is cadmium sulfide, the pervious layer 17 may preferably comprise a layer of alumina approximately one-eighth to oneahalf inch thick. It should be understood that the thicker the layer of inert material 17, the slower would be the evaporation of the subliming material while a thinner layer of inert material would be less eifective to quiet the motions of the subliming material and provide the desired filtering action by the aforementioned repeated condensing and evaporating actions as the subliming material percolates through the pervious layer. The crucible 15 may be heated to a temperature in the range of 650 to 1,000 centigrade when causing cadmium sulfide to sublime and it is thus apparent that the temperature of sublimation is not very critical since the sublimation step is believed to be only speeded up by increases in temperature. When alumina is used as the particulate inert material, it should preferably have a particle size greater than the particle size of the cadmium sulfide and may be about one hundred times greater than the particle size of the cadmium sulfide. Also it is to be noted that the alumina is a hard, refractory material that is relatively inert and stable up to temperatures in the vicinity of 2050 centigrade which is considerably higher than the temperature Within the crucible container 15 as required to cause the effective sublimation of the cadmium sulfide. When evaporating cadmium sulfide, the partial vacuum Within the enclosure provided by the bell jar 12 and the vacuum plate 10 may be in the order of 10- to millimeters of mercury and may be of a conventional atmospheric gaseous composition.

Obviously, the method of the invention is not limited to the vacuum evaporation coating technique as used to deposit a film of cadmium sulfide on a desired surface although such technique has been explicitly described as one example of a practical application of the method of the invention. Other examples of particulate subliming marterials that may be used in vacuum coating techniques employing the method of the invention are zinc sulfide and silicon monoxide. Although either of these substances requires a higher temperature range for efiective sublimation than cadmium sulfide requires, the particulate inert material used may still be alumina since the alumina will not decompose or otherwise change its form at the higher ranges of temperature as required for sublimation of these materials. Other particulate inert materials in place of alumina may be used so long as they are inert at the required temperatures of sublimation and do not change their particulate form. For example, other refractory materials such as zirconium oxide or thorium oxide may be used as the particulate inert layer material.

Various modifications will occur to those skilled in the art within the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. A method for evaporating particulate subliming material for the purpose of coating a surface comprising, placing a quantity of particulate subliming material in a container, placing on said subliming material a contiguous pervious layer of inert particulate material having higher decomposition, melting, and evaporating temperatures than the temperature of evaporation of said subliming material, and heating the container and its contents to a temperature sufficient to evaporate said subliming material through said contiguous pervious layer.

2. A method for evaporating particulate subliming material for the purpose of coating a surface comprising, placing a quantity of particulate subliming material in a container, placing in contact with said subliming material a contiguous per-vious layer of inert particulate material having an average particle size greater than the average particle size of said subliming material and higher decomposition, evaporating, and melting temperatures than the temperature of evaporation of the subliming material, and heating the container and its contents to a temperature suificient to evaporate said subliming material through said contiguous pervious layer.

3. A method for evaporating particulate subliming material for the purpose of coating a surface comprising, placing a quantity of particulate subliming material in a container, placing in contact with said subliming material a contiguous pervious layer of inert particulate material having higher decomposition, evaporating, and melting temperatures than the temperature of evaporation of said subliming material, and heating the container and its contents in a partial vacuum to a temperature sufficient to evaporate said subliming material through said contiguous pervious layer.

4. A method for evaporating particulate subliming material for the purpose of coating a surface comprising, placing a quantity of particulate subliming material in a container, placing in contact with said subliming material a contiguous pervious layer of inert particulate material having an average particle size greater than the average particle size of said subliming material and higher decomposition, evaporating, and melting temperatures than the temperature of evaporation of said subliming material, and heating the container and its contents in a partial vacuum to a temperature sufiicient to evaporate said subliming material through said contiguous pervious layer.

5. A method for evaporating particulate subliming material for the purpose of coating a surface comprising placing a quantity of particulate subliming material in a container, adding on said quantity of particulate subliming material inert particulate material having higher decomposition, melting, and evaporation temperatures than the temperature of evaporation of said subliming material, and heating the container and its contents to a temperature suflicient to evaporate said subliming material.

62. A method for evaporating subliming materials for the purpose of coating a surface the steps comprising, placing in a container a quantity of particulate material selected from the group of subliming materials consisting of cadmium sulfide, zinc sulfide, and silicon monoxide, adding on said quantity of particulate subliming material a layer of inert particulate material selected from the group of inert materials consisting of aluminum oxide, zirconium oxide and thorium oxide, and heating said container and its contents to a temperature sufficient to evaporate said particulate subliming material.

References Cited in the file of this patent UNITED STATES PATENTS 1,353,521 Duodo Sept. 21, 1920 1,987,282 Comte Jan. 8, 1935 2,271,023 Nelson Jan. 27, 1942 2,398,382 Lyon Apr. 16, .1946

2,754,178 Mack July 10, 1956 FOREIGN PATENTS 78,6111 Netherlands June 22, 1955 OTHER REFERENCES Classification Def, US. Pat. Ofiice, March 1950, pages 202-2l8. 

1. A METHOD FOR EVAPORATING PARTICULATE SUBLIMING MATERIAL FOR THE PURPOSE OF COATING A SURFACE COMPRISING, PLACING A QUANTITY OF PARTICULATE SUBLIMING MATERIAL IN A CONTAINER, PLACING ON SAID SUBLIMING MATERIAL A CONTIGUOUS PERVIOUS LAYER OF INERT PARTICULATE MATERIAL HAVING HIGHER DECOMPOSITION, MELTING, AND EVAPORATING TEMPERATURES THAN THE TEMPERATURE OF EVAPORATION OF SAID SUBLIMING MATERIAL, AND HEATING THE CONTAINER AND ITS CONTENTS TO A TEMPERATURE SUFFICIENT TO EVAPORATE SAID SUBLIMING MATERIAL THROUGN SAID CONTIGUOUS PERVIOUS LAYER. 